The present invention relates to measuring devices, and in particular to thickness measuring devices for measuring the thickness of sheet material such as plastic, paper, fabric, non-woven textile and the like. Many sheet materials, such as metal foil, rolled sheet metals, and hard or highly crosslinked polymer sheet, are readily measured with a high degree of repeatability using simple hand instruments such as screw-threaded micrometers or dial micrometers, or by using automated machine measurement instruments. By contrast, compliant or soft sheet materials often are made of fibrous matter so that they have a poorly defined edge extent and possess a fuzzy band of matter extending outside of the nominal plane of the sheet, making mechanical and even optical measurement equipment inappropriate; also when a major portion of their body is soft and compressible this may make measurement repeatability problematic. By way of example, rag papers and particularly thicker rag papers such as lithographic and etching papers may possess both surface roughness and a compressibility which, moreover, may vary considerably depending on environmental factors such as relative dryness due to the ambient air.
Traditionally, such materials are best measured by placing a stack of sheets, 5, 10, or 50 layers thick, between the sensing jaws of a measurement apparatus, and applying a specified compressive load for a certain period of time, before taking a measurement. This eliminates the high degree of variation caused by the relatively compressive fibrous face on the external surface of a single sheet, and averages the bulk compressibility over a large number of sheets to get a more representative measurement. A number of related protocols have been developed for obtaining standardized and ostensibly more accurate measurements for such compressible materials.
It will be appreciated, however, that a measurement which involves an average over many sheets of material taken in a compressed form under static conditions, while tending to produce a definite number, is a highly artificial measurement. Thus, it is possible for two sheets of substantially different compressibility and different fibrous make-up, to have the same nominal thickness when measured by a compression/stabilization protocol of this type. In practice, thickness measurements on compressible or non-woven webs of materials may be useful to understand the behavior--e.g., image-receptive properties, wetability etc.--of the outer surface, which is precisely the portion that is compressed and not measured in a standard protocol. Thickness measurements may also be necessary for the adjustment and proper operation of complex machinery which employs web material, such as printing, binding, calendaring, or surface treatment machinery in which adjustment of the spacing between opposed rollers, adjustment of the load exerted by a roller, or more complex adjustments involving tensioning mechanisms, inking mechanisms or processing machinery adjustments, are to be optimized based on the thickness of only a single sheet of material passing through in its generally uncompressed, or instantaneously compressed state. Thickness measurements may also be desirable in a paper-manufacturing process line, in which detected characteristics may be used to adjust processing machinery or to alter the feedstock. Thus, it would be highly desirable to develop a measuring instrument capable of performing a thickness measurement on a single sheet or web of compressible or springy material, which dependably and repeatably measures the actual thickness of the material rather than a net or average thickness of the material after a preconditioning step or interaction with the measurement device.